In a chemical laboratory or similar environment it is frequently desirable to use a robot or other material handling apparatus for sample preparation and transport between instruments. Using such apparatus permits manipulation while caustic or other hazardous reactions are taking place, and also permits the handling of otherwise dangerous chemicals and their containers at extremes of temperature or other hazardous conditions. It is important that the robot or other material handling apparatus used in these applications is capable of gripping various shaped vessels containing the samples. Among the most common of these vessels are glass test tubes; however, numerous others such as small flasks, beakers, vials, bottles with screw caps or crimp caps, and plastic trays which contain many individual sample wells, the latter known as microtiter plates, are also commonly handled. Moreover, many laboratory procedures are repetitive and require the handling of several different vessels in a sequence; therefore, the laboratory robot or other material handling apparatus must be able to readily change its "end effectors" (i.e., "grippers" or "fingers") during the execution of such a sequence.
Numerous end-effector exchange mechanisms have been proposed by others. For example, U.S. Pat. No. 4,549,846--Torri et al. discloses a hand exchanger for industrial robots, which has a changer unit mounted fixedly on the industrial robot and a changer adapter holding a robot hand which is detachably fitted into the changer unit. The changer unit has hydraulic piston end clamping balls urged toward a clamping position by a piston. The changer adapter is fitted in the changer unit and clamped by the agency of the clamping piston. Another exemplary device is described in U.S. Pat. No. 4,621,854--Boley et al. This patent discloses a device for changing tools in an industrial robot and consists of a linear drive mechanism which drives a hook-shaped structure to initially carry out a curved motion, essentially perpendicular to the linear closing or opening motion of the hand. These devices can be classed among those that contain active elements such as electromagnets or hydraulic actuators to attach the end effector to the rest of the apparatus.
A second class of end-effector exchange mechanisms involves those without active elements which employ the robot's own motion capability to pick up, engage, unengage and deposit the tools. One example of such a device is disclosed in U.S. Pat. No. 4,660,274--Goumas et al. This patent discloses a robot tool changing apparatus which effects the transfer and connection of tools between a tool holder and a robot hand. A pin on the tool holder and a pin on the hand are received during tool transfer within a bore extending through the tool shank. One of the pins enters the bore from one end thereof and the other pin enters from the other end of the bore.
A "passive" gripper exchange mechanism is also disclosed by U.S. Pat. No. 4,715,636--Wiesner et al. This device provides a circular coupling opening and a locking member opening of rectangular cross-section disposed transversely across the circular coupling opening. The gripper or finger to be exchanged is provided with a coupling plug having a lateral, circumferential recess in the form of an annular groove. When the device is in the "locked" state, a locking member engages both the locking member opening and the lateral recess of the coupling plug. The locking and unlocking of the gripper from the robotic arm is accomplished by sliding surfaces which the locking member is moved and urged against by the gripper actuation mechanism. The Wiesner et al. reference uses a rigid deposit fixture plate to restrain the grippers during the engagement or release of the locking member and coupling plug.
The apparatus disclosed by Wiesner et al. still suffers from certain limitations, however, particularly in reference to its use in laboratory robots having relatively small operating forces and otherwise relatively delicate construction. The apparatus disclosed requires the coupling plug to be axially spring loaded to resist the lateral movement of the locking member. This movement is caused by the locking member being forced over a wedge surface during locking and release movements. The coupling plug must necessarily be provided with sufficient clearance with respect to the coupling member to prevent it from jamming. The reference therefore provides resilient catches and locking members to accommodate the clearance once locking is achieved. Moreover, the clearance required by the spring loaded coupling plug of Wiesner, et al. prevents it from serving as an accurate positioning device for the gripper/arm interface. Thus, the referenced device requires two sets of conical plugs, cylindrical pins, cone-shaped receptacles and a synthetic resin fill-up to achieve registration in the locking sequence.
Therefore, the apparatus disclosed presents a device requiring a relatively large force to operate and overcome the various spring loaded members and other forces in its interfaces, while also having an inherently inaccurate registration between the coupling plug and coupling opening which must be "taken up" by other means. These attributes do not lend themselves well to reliable and accurate tool exchange using the relatively small forces provided by laboratory robots and similar materials handling apparatus used in high precision applications.